Just began to experience a loss of power, climbing hills. Everything seems fine on straightaway. This is 3.6L. It almost feels like emergency brake is stuck on, but this is not the case. I checked plugs, #6 has oil but not below the threads. I am about to check air flow sensor (possible dirt?) or throttle position sensor. Engine runs smoothly, idles well, etc. I am continuing to explore but would appreciate help from anyone with better ideas.

Thanks for the reply. I checked with my exhaust mechanic who I have used successfully in the past (though not with my Jaguar) and he said there is no reliable way other than replacement or dynomometer to test the cats???? I decided to test the fuel pressure, I have a gauge, but with only barb connection. Is there a special tool to adapt to what looks like a test port at the front of the fuel rail? I am reluctant to break into the fuel line to attach my gauge.

You can test the exhaust back pressure by simply removing the oxygen sensor and fabricating an adapter to connect a low pressure gauge to the port. Granted, it's easier to say than to do, but sometimes some effort is required. At idle, the pressure should be around 1 psi. At 2000 RPM it would be around 3 psi. If more than that, a catalyst restriction is indicated.